Method of applying an automotive type oxygen sensor for use in an industrial process analyzer

ABSTRACT

An industrial process oxygen analyzer, and method of using same, incorporates an automotive oxygen sensor to minimize the criticalness of controlling heater temperature during operation. Higher sensor heater temperature, required to accommodate the automotive oxygen sensor, is achieved by application of higher supply voltage to the sensor heater. Current limiting of the heater supply circuit reduces stress on the sensor heater, even at the higher operating temperature.

This is a division of application Ser. No. 07/092,868 filed Sept. 3,1987 now abandoned.

TECHNICAL FIELD

The present invention relates to the use of an oxygen sensor inindustrial process control and more particularly to the use of anautomotive type oxygen in an industrial process analyzer.

BACKGROUND ART

Oxygen analyzers are used extensively in industrial process control.When used in such control applications, these analyzers typicallyincorporate three major components--a zirconium dioxide sensor whichproduces a voltage output signal representative of oxygen concentrationwithin the gas sample being analyzed, a heater to elevate thetemperature of the sensor to a required operating temperature, and aheater control circuit to maintain the sensor temperature independent ofenvironment. It has been found that the use of a typical zirconium oxidesensor for industrial process control has a number of inherentdisadvantages. For example, typically the use of such a sensor requiresthe utilization of many associated parts, clamps, seals and fastenersmaking assembly and/or replacement a difficult task. In addition, it hasbeen found that tight gas seals are difficult to maintain. Also,alignment of the components comprising the assembly is difficult toachieve and maintain. The zirconium dioxide sensor must be located inthe gas stream and the heater and thermocouple must be aligned with thesensing tip. It has been further found that exposure of the heaterelement to the corrosive gas stream reduces heater life. And lastly,complex heater control circuitry is required because of the low mass ofthe heater. Temperature control is critical to prevent deviations of thesensor output.

Because of the foregoing, it has become desirable to develop asimplified system for analyzing the oxygen content of a gas utilized inan industrial process. Such a simplified system should minimize thecriticalness of controlling heater temperature within the sensor.

SUMMARY OF THE INVENTION

The present invention solves the problems associated with the prior artand other problems by utilizing an autotive type oxygen sensor in placeof the costly zirconium dioxide sensor typically used to determine theoxygen content of a gas, in an industrial process. The use of such anautomotive type oxygen sensor requires a higher operating temperaturethan that normally required for a zirconium dioxide sensor, but sensorheater temperature is not nearly as critical. Such a higher operatingtemperature is achieved by applying a higher supply voltage to thesensor heater than typically used and by utilizing a current limitingcircuit in conjunction with the higher supply voltage. The use of ahigher supply voltage does not decrease heater life since current levelsare actually less than that experienced with a lower supply voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the use of an automotive typeoxygen sensor in conjunction with an associated power supply to analyzethe oxygen content of a gas in an industrial process.

FIG. 2 is an electrical schematic drawing of the power supply for theautomotive type oxygen sensor utilized to analyze the oxygen content ofa gas in an industrial process.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings where the illustrations are for thepurpose of describing the preferred embodiment of the present inventionand are not intended to limit the invention hereto, FIG. 1 is aschematic diagram of a system 10 for analyzing the oxygen content of anindustrial process by using an automotive type oxygen sensor 20. In thissystem 10, a gas sample is drawn from the monitored industrial processthrough a sample probe, shown generally by the numeral 12. The drawingof this sample is typically accomplished through the use of an airpowered aspirator (not shown) within the oxygen analyzer. The gas sampleis directed through a passageway 14 in the analyzer manifold 16 acrossthe sensor element 18 of the automotive type oxygen sensor 20 and isexhausted back into the gas flow within the industrial process. Theanalyzer manifold 16 is controlled at a substantially constanttemperature above the gas stream dew point, typically 300 degrees to 600degrees F. (149 degrees to 316 degrees C.). The controlled manifoldtemperature provides a substantially constant ambient temperature forthe automotive type oxygen sensor 20. The analyzer manifold 16 is heatedby heaters 22 and controlled by a temperature sensing element 24 whichis connected to a temperature control circuit 26. An integral heater(not shown) within the automotive type oxygen sensor 20 is connected toa power supply 30 which is adjusted to provide the desired operatingtemperature at the sensor element 18 of automotive type oxygen sensor20. The operating temperature of the sensor element 18 is dependent uponthe process being monitored but is typically between 1300 degrees to1500 degrees F. (704 degrees to 816 C.).

When an automotive type oxygen sensor 20 is used, the temperature of thesensor element 18 is not critical since sensor is used only as a switchat or near stoichiometric conditions (excess air factor λ=1). The switchoccurs in the range of 200 to 500 mv output and can operate reliablyanywhere above 350 degrees C. Analyzing industrial processes for percentoxygen content requires accurate measurements above the λ=1 controlrange to 100% excess air. Higher operating temperature is required toraise the mvdc output level to a usable span over this range and also tominimize the effect of interfering gases such as sulfur dioxide. Thishigher operating temperature is achieved by applying a higher supplyvoltage to the heater within the automotive type oxygen sensor 20 whilelimiting the current thereto. Such a higher supply voltage is providedby the power supply 30 as hereinafter described.

Referring now to FIG. 2 which is an electrical schematic of power supply30, this power supply converts 120 volts AC 50/60 Hz AC line power tothe 18 VDC, 1 amp level required by the heater within the automotivetype oxygen sensor 20. This power supply also limits the currentsupplied to the heater under cold start-up conditions. This power supply30 includes a transformer 32 which converts the 120 volts AC line inputto a voltage which is somewhat greater than the voltage typicallyutilized by the sensor heater. A bridge rectifier 34 and capacitor 36convert the AC input to the transformer 32 to a DC voltage which isapplied to a voltage regulator 38 which, in turn, regulates the DCvoltage to a level determined by a series of resistors 40, 42 and 44.Capacitor 46 is connected across the combination of resistors 40, 42 and44 to provide additional filtering and stability to the circuit. Thecombination of resistors 40, 42 and 44 allows the output voltage of thecircuit to be varied according to installation requirements.

During normal operation, the power supply 30 supplies the nominal 18volts DC and 1 ampere power required by the sensor heater. During coldstart-up conditions, a current limiting mechanism, which is an integralpart of voltage regulator 38, varies the output voltage of the powersupply 30 so that the maximum current through the sensor heater does notexceed approximately 2.2 amps.

Controlling sensor heater voltage through the output voltage of thepower supply 30 provides several distinct advantages. First, the heatervoltage is controlled much more closely than in typical automotiveapplications thus making the oxygen concentration measurementconsiderably more accurate. In addition, the voltage which is applied tothe heater is somewhat higher than the nominal 12 to 14 volts typicallyused in an automotive application allowing the heater to reach a higheroperating temperature permitting the automotive type oxygen sensor to beused in an industrial process control application. And lastly, thecurrent limiting ability of the power supply reduces the stress on theheater during cold start-up conditions.

The possibility of using an automobile type oxygen sensor in anindustrial process control application provides several inherentadvantages. For example, the oxygen sensor 20 threads directly into theanalyzer manifold 16 and seals with only one captive spark plug typeseal. No alignment is required with respect to a separate heater,thermocouple or gas stream as is typically required with presentindustrial oxygen analyzers. By using an automotive type oxygen sensor,the integral internal heating element is not exposed to corrosiveprocess gases, thus extending heater life. The positive temperaturecoefficient of the heater element used in an automotive type oxygensensor aids is current limiting of the heater as the operatingtemperature is approached. Current limiting of the heater supply circuitprovides cold start-up current protection thus allowing for highervoltage operation (18 volts DC typical) than in automotive applications(13.5 volts DC typical). This, in turn, requires less ambienttemperature control (300 degrees F. typical) for the analyzer manifoldto obtain the required operating temperature (1300 degrees F. typical)for the heater. Temperature control of the higher mass, lowertemperature analyzer manifold, instead of the smaller sensor heater,simplifies the heater control circuitry. And lastly, the more ruggeddesign of the automotive type oxygen sensor greatly reduces thepossibility of breakage of time.

Certain modifications and improvements will occur to those skilled inthe art upon reading the foregoing. It should be understood that allsuch modifications and improvements have been deleted herein for thesake of conciseness and readability, but are properly within the scopeof the following claims.

I claim:
 1. A method of analyzing the oxygen content of a gas,comprising the steps of:situating an automotive type oxygen sensor in ananalyzer manifold, the automotive type oxygen sensor having a sensorelement and integral electric heating means; maintaining the sensorelement within a predetermined operating temperature range by applying avoltage to the integral electric heating element of the automotive typeoxygen sensor, said voltage being greater than about 12 volts; directinga gas sample through the analyzer manifold to the sensor element of theautomotive type oxygen sensor to obtain an output millivolt D.C signaltherefrom indicative of the oxygen content of the gas sample; andlimiting the current applied to the automotive type oxygen sensor duringstart-up to extend the life of the sensor.
 2. The method according toclaim 1, wherein the predetermined operating temperature range of thesensor element is between 1300 degrees F. and 1500 degrees F.
 3. Themethod according to claim 1, further including the step of maintainingthe temperature of the analyzer manifold at a substantially constanttemperature above the dew point temperature of the gas sample.
 4. Themethod according to claim 1, wherein the voltage applied to the integralheating element of the automotive type oxygen sensor is about 18 voltsD.C. and the current level is about 1 ampere during normal operation. 5.The method according to claim 1, wherein the maximum current through theintegral heating element is initially limited to not exceedapproximately 2.2 amperes during start-up conditions.
 6. A method ofanalyzing the oxygen content of a gas, comprising the steps of:situatingan automotive type oxygen sensor in an analyzer manifold, the automotivetype oxygen sensor having a sensor element and integral electric heatingmeans; maintaining the sensor element within a predetermined operatingtemperature range by applying a voltage to the integral electric heatingelement of the automotive type oxygen sensor, said voltage being greaterthan about 12 to 14 volts; directing a gas sample through the analyzermanifold to the sensor element of the automotive type oxygen sensor toobtain an output millivolt D.C. signal therefrom indicative of theoxygen content of the gas sample; and limiting the current initiallyapplied to the automotive type oxygen sensor for extending the life ofthe sensor.
 7. A method of analyzing the oxygen content of a gas,comprising the steps of:situating an automotive type oxygen sensor in ananalyzer manifold maintained at a constant temperature of about 1300° F.to about 1500° F., the automotive type oxygen sensor having a sensorelement and integral electric heating means; maintaining the sensorelement within a predetermined operating temperature range by applyingapproximately 18 volts D.C. to the integral electric heating element ofthe automotive type oxygen sensor; directing a gas sample through theanalyzer manifold to the sensor element of the automotive type oxygensensor to obtain an output millivolt D.C. signal therefrom indicative ofthe oxygen content of the gas sample; and limiting the current appliedto the automotive type oxygen sensor during start-up for extending thelife of the sensor.